I've just used MKVExtractGUI-2 to extract the subtitles (.ass format) from .mkv file and edit it. But when I merged the edited subs back into .mkv file with MKVToolnix and then played it with WMP 12, the subtitles won't display. This is very strange since I had no issue displaying the subs from the original .mkv file and it also worked with the separate subs (in the same directory). It should work when merging them, yet it didn't. I wonder what caused this problem?

Any help is appreciated.